@uberk I have almost the exact same set up as you. Amati G5's biamped with MC451 monoblocks. My difference is only that I have an AVM90 vs a MX123.
So now that I've actually added the C12000 I can comment on how it works/how its set up.
The L/R output from your MX123 would be plugged into the "passthrough" input, the L/R output goes to your amps on the C12000.
If you turn on the C12000 your amps turn on, you have an awesome two channel system. If you turn on your MX123, the C12000 turns on, the amps turn on and the system immediately goes to "passthrough" mode. Everything works as your existing home theater set up does - the controls on the C12000 are not functional even though its is on.
Works perfectly!
Hope that helps.
PB